Food and beverage can bodies and ends often are made from metal sheeting that has been precoated with a polymer coating to prevent contact of the metal with the contained food or beverage. Defects often appear in heavily fabricated areas of the can bodies and ends. If not repaired, such defects may lead to corrosion and potential spoilage. Coating defects in these metal parts customarily are repaired by spraying, dipping or electrocoating the parts; of these methods, the electrocoating process is the most efficient because it preferentially coats in the area of defects.
Electrocoating compositions commonly include polymers made from derivatives of methacrylic acid, acrylic acid and styrene, and compositional ranges of these materials approved for contact with food are set out in 21 CFR 175-300. Examples of such compositions are found in U.S. Pat. Nos. 4,308,121 (Hazan) and 3,939,051 (Anderson et al.). Other coating compositions used for electrocoat repair have employed minor amounts of epoxy ester resins in combination with maleinized oils. These coating materials are not completely satisfactory because they exhibit less than optimum resistance to aggressive foods such as sauerkraut and tomato products.
U.S. Pat. Nos. 4,598,109 and 4,425,451 (both Sekmakas, et al.) refer to water-dispersible epoxy-phosphate ester polymer salts that are used in combination with aminoplast or phenoplast resins. In these references, an epoxy resin is reacted with less than a stoichiometric quantity of phosphoric acid to provide an epoxy phosphate ester, the remaining oxirane groups of which are reacted with a volatile amine to provide a water-dispersible product lacking oxirane functionality. U.S. Pat. No. 4,461,857 (Sekmakas, et al.) shows a similar coating material which includes from 25-85% of a carboxyl-functional, organic solvent-soluble copolymer salt with a volatile amine. U.S. Pat. No. 4,397,970 (Campbell, et al.) describes an improved process for preparing epoxy resin/phosphoric acid reaction products which are manufactured using an intermediate blocking agent. U.S. Pat. No. 4,164,487 (Martin) refers further to coatings containing water-thinable, base-neutralized phosphoric acid/polyether epoxide reaction products employing two or more epoxy resins.